Structure of diatomic molecules

Molecules are of course atoms that are held together by shared valence electrons. That is, most of each atom is pretty much as it would be if the atom were isolated, but one or a few electrons are located in regions (for example, between two atomic cores) where they lead to an overall attractive e®ect. To see roughly how this works, you might consider the case of two protons and two electrons at the corners of a square. If you calculate (classically) the total electrostatic energy of this arrangement, you will ̄nd that it is negative { the system is bound. Molecules have several kinds of motion, with quite di®erent energy scales. Assume that a couple of nuclei and a couple of electrons are con ̄ned to a volume with characteristic size a (a couple of oA, typically). Then the uncertainty principle requires that at a minimum, p » 1 h=a, so the typical electron energies will be of order